the periodontal ligament is already decayed and has turned into bacteria, which are munching on your jawbone.  how bad this has gotten depends on your immune system and length of time after the tooth was pulled.  the dr doesn't cut through the bone, because if there was bone there, there would not be a problem.  in other teeth about 95% pulled by a regular dentist form cavitations, but in wisdom teeth it is near the 100% mark because of the size and trauma to the area.

A cavitation is an unhealed hole in the jawbone caused by an extracted tooth. Since wisdom teeth are the most commonly extracted teeth, most cavitations are found in the wisdom tooth sites. Please see the graphic and photo below to get a glimpse of what may be in your mouth and the effects it is having. The photo and diagram demonstrate the destructive and pathologic consequence of a routine tooth extraction. Dentists are taught in dental school that once they pull a tooth, the patient's body heals the resulting hole in the jawbone. However, approximately 95% of all tooth extractions result in a pathologic defect called a cavitation. The tooth is attached to the jawbone by a periodontal ligament which is comprised of "jillions" of microscopic fibers. One end of each fiber is attached to the jawbone and the other end of the fiber is attached to the tooth root. When a tooth is extracted, the fibers break midway between the root and the bone. This leaves the socket (the area where the root was anchored in the bone) coated with periodontal ligament fibers. There are specialized cells in the bone called osteoblasts. Osteoblasts make new bone. The word "osteoblast" means bone former. They are active during growth and maintenance. However, the periodontal ligament prevents the osteoblasts from filling in the tooth socket with bone since the periodontal ligament fibers lining the socket act as a barrier beyond which the osteoblasts cannot form bone. In other words, an osteoblast "sees" a tooth when it "sees" periodontal ligament fibers. Since there are billions of bacteria in the mouth, they easily get into the open tooth socket. Since the bone is unable to fill in the defect of the socket, the newly formed "cavitation" is now infected. Since there is no blood supply to the "cavitation" it is called "ischemic" or "avascular" (without a blood supply). This results in necrosis (tissue death). Hence we call a cavitation an unhealed, chronically infected, avascular, necrotic hole in the bone. The defect acts to an acupuncture meridian the same way a dead tooth (or root canal tooth) acts. It causes an interference field on the meridian which can impair the function and health of other tissues, organs and structures on the meridian. Significantly, the bacteria in the cavitation also produce the same deadly toxins that are produced by the bacteria in root canals (see Root Canals). These toxins are thio-ethers (most toxic organic substance known to man), thio-ethanols, and mercaptans. They have been found in the tumors in women with breast cancer.